Making Sense of Health Statistics

Politicians use them to persuade voters, drug companies to sell products, and the media spin them in all kinds of ways, but few really understand health statistics. Even most doctors, who must make daily decisions based on numerical data - for instance, to calculate the risks of a drug or surgical intervention - lack the basic statistical literacy required.

The problem of statistical illiteracy has two sides, the researchers say. It is caused by the confusing ways statistics are ordinarily presented in health communication, coupled with a lack of statistical thinking skills among consumers of health information.

The combination can be explosive. A mid-1990s report in Britain showed that new oral contraceptive pills posed a twofold (or 100 percent) increased risk of blood clots led to a panic among women. Many stopped using this form of birth control. There were an estimated 13,000 more abortions in England and Wales in the year following the report. Had the public been informed that the absolute risk with the new pills only increased from one in 7,000 women having a blood clot to two in 7,000, the panic would not have occurred.

Health statistics are often presented in relative risks (percentages), which are generally large numbers that capture people's attention. Percentages are misleading though when facts like base rates are left out. Absolute risk figures (like “one in 7,000") are much more understandable, and less easy to sensationalise.

The authors of the report also cite studies showing the inability of doctors to accurately interpret data from cancer screenings or other test results, when the data were presented in the typical form of conditional probabilities. For example, when presented with conditional probabilities, a group of experienced physicians was unable to accurately estimate the chances a patient with a positive test result actually had colorectal cancer, given the known sensitivity and false-positive rate of the test used (estimates ranged from a one percent to a 99 percent chance, with most guessing around 50 percent). Most physicians gave the right answer (a less than ten percent chance) when the data were presented in the more transparent form of natural frequencies.